Oxalic Acid (H_2C_2O_4) is a diprotic acid. What is the pH of a 0.200 M solution

Question

Oxalic Acid (H_2C_2O_4) is a diprotic acid. What is the pH of a 0.200 M solution of oxalic acid? Consider the contribution from both the protons when calculating the pH. Look up the relevant K, values from your text book. Draw a rough sketch that shows the titration curve when 100. mL of this solution (0.200 M Oxalic acid) is titrated with 0.200 M NaOH solution. Label the titration curve clearly, Indicate the regions on the titration curve where there is a(n) a) weak acid b) Buffer c) Half equivalence point (show all if there is more than one) d) Equivalence point (show all if there is more than one) Calculate the pH of the solution when 100 mL of NaOH has been added.

Explanation / Answer

H2C2O4 « H+ +H C2O4-

0.020           0        0

0.020-x        x       x

Ka1 = [H+] [H C2O4-] / [H2C2O4 ] = 5.9 x 10-2

x2 / 0.020-x = 5.9 x 10-2

x = [H+] = 0.0158

pH = -log 0.0158 = 1.80

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